1. Field of the Invention
One or more embodiments setting forth the ideas described throughout this disclosure pertain to the field of antennas. More particularly, but not by way of limitation, one or more aspects of the disclosure enable a directional planar log-spiral slot antenna.
2. Description of the Related Art
Some buildings are difficult to receive cell phone coverage in. In order to provide cell phone coverage in these buildings, “microcells” have been installed to accommodate cell phone calls within these buildings. Microcells are implemented as low power cell instances in a mobile phone network wherein the microcells have a correspondingly low coverage area with respect to a standard cell. Typical microcell range is under a mile in radius. Current implementations of microcells suffer from the types of antennas that are utilized. These microcells are generally omnidirectional antennas that allow access from locations external to the building in which they are installed.
Use of wide band antennas allows for the cell or microcell sites to handle more phone calls. Thus, wide band antennas are generally used for cell antennas. Walls in buildings begin to attenuate the cell phone signals at around 1.8 GHz and thus although coverage is limited above this frequency, the number of calls that may be handled is actually higher as individual floors within a building may simultaneously use the same frequency for separate phone calls.
Another approach to increase coverage within buildings uses bi-directional amplifiers (BDAs) to boost the cell phone coverage by receiving cell phone signals, amplifying them and retransmitting the signals, for example into/out of a building and from/to an existing cell tower for example. This solution may be cheaper and easier to implement than a full fledged microcell, and is expected to form a large portion of the market for increasing cell phone coverage for example within buildings. These installations utilize antennas within the building and an antenna generally on the outside of a building for example an externally directed antenna, e.g., pointed in the direction of the highest power cell tower within range.
For microcells and BDAs, it is desirable to primarily employ coverage within the building itself and not outside the building where the cell towers can handle traffic. With better antennas covering the inside of a building, fewer antennas are required as well.
One type of wide band antenna is a planar logarithmic-spiral antenna, also known as a planar log-spiral antenna. Log-spiral antennas have been known at least since 1955. Two-arm Log-spiral antennas generally have two spiral shaped arms shifted 180 degrees from one another. This type of antenna yields left hand circularly polarized radiation in one direction away from the plane, and right hand circularly polarized radiation in the opposite direction. Attempts at absorbing rearward pointing energy in order to make a directional antenna have been less than optimal in that known directional implementations still have sizable back lobes. This means that in the case of a BDA, the antenna pointing into the building would still have a fairly high gain pointing out of the building, which can cause problems with the externally directed antenna for example.
Since cell phones are generally linearly polarized, if wideband linearly polarized antennas are utilized within a building, then when a cell phone tips, i.e., the cell phone antenna is displaced out of a vertical orientation, for example when a cell phone user leans back in a chair, the signal fades. This is known as polarization fade. Circularly polarized antennas, for example coupled with a microcell or BDA, may receive the linearly polarized energy from the cell phone and do not experience this type of fade.
Another application for wideband antennas is for gathering signals for intelligence, i.e., “SIGINT”. Wideband antennas for example are utilized in running continuous searches for signals or scanning known frequencies that may extend over a large range. In addition, in the intelligence world, one antenna can be utilized for many purposes. For covert communications, if a transmitter with a wide band antenna is captured for example, it is impossible to tell exactly what frequency the system was operating at. Also, if the antenna has a wide range of operation, e.g., 700 MHz-2500 MHz, there is no way to determine what the antenna was being used for.
For at least the limitations described above there is a need for a directional planar log-spiral slot antenna with low back lobes.